The measurement of certain properties of a medium can be effected by using acoustic waves. For this measurement method, which is described for example in WO 2008/034878 A2 or WO 2010/136350 A1, the medium is in direct contact with an acoustic waveguide. In the waveguide acoustic surface waves are excited, wherein the type and frequency of the surface waves are chosen such that a partial coupling out into the medium is effected. A part of the acoustic surface waves in the waveguide therefore is coupled into the medium as longitudinal volumetric sound waves. These sound waves pass through the medium and are reflected on a surface bordering the medium such that they again impinge on the waveguide. There, a part of the volumetric sound waves again is coupled into the waveguide as acoustic surface waves and runs on in the same. At an acoustic receiver, which is arranged at the waveguide spaced from the transmitter, a characteristic signal thereby is obtained, whose temporal course of intensity including the time delay with respect to the signal sent by the transmitter permits conclusions regarding characteristic properties of the medium such as sound velocity, temperature, homogeneity, flow velocity, flow rate, concentration or viscosity.
This measurement method is suitable in particular for liquid, but also for highly viscous, dough-like, gel-like or pasty media of homogeneous or inhomogeneous nature, including biological samples. The use for gaseous media would also be conceivable. When the medium flows through the device, temporal changes of the medium can also be detected.
The spatial propagation of the volumetric sound waves in the medium for example is achieved in that the volumetric sound waves are coupled out into the medium at an angle δ based on a surface normal of the waveguide. The relation can be described by the following formula:
      δ    =          arcsin      ⁡              (                              c            M                                c            S                          )              ,
wherein cM is the sound velocity of the volumetric sound waves inside the medium and cS is the sound velocity of the acoustic surface waves propagating along the waveguide.
In the most frequent case, in which the sound velocity in the medium is smaller than that of the surface waves in the waveguide, sound waves are coupled out at an angle different from 0, and the volumetric sound waves cover a spatial distance along the waveguide, possibly by multiple reflection within the medium.
In the known devices, transmitter and receiver are mounted on a side of the waveguide opposite the interface with the medium. To ensure that surface sound waves excited on this side of the waveguide can be coupled into the medium, Lamb waves therefore preferably are excited, i.e. waves whose wavelength is much longer than the thickness of the waveguide between transmitter and medium. In this case, both the upper and the lower side of the waveguide are moving, wherein the vibration also has a longitudinal component. Therefore, this type of excitation is suitable for coupling out volumetric sound waves. It is also possible to chose the wavelength of the excited acoustic surface waves in the order of magnitude of the thickness of the waveguide, wherein then surface waves are excited in a transition region between Lamb waves and Rayleigh waves.
The devices described so far in the prior art, which operate by the principle described above, have a complex construction and are expensive to manufacture.
Therefore, it is the object of the invention to create a simply constructed and easily manufacturable device for determining properties of a medium by means of the excitation of surface sound waves.